(a) Technical Field
The present invention relates to a sealing glass composition for an intermediate-temperature planar solid oxide fuel cell (SOFC).
(b) Background Art
Extensive researches have recently been exerted for the commercialization of an intermediate-temperature SOFC having an operation temperature of 600-800° C. The intermediate-temperature SOFC has many advantages over a high-temperature SOFC having an operation temperature of 800-1000° C. For example, while a ceramic interconnector is required for the high-temperature SOFC, a metallic interconnector, which is cheaper than the ceramic interconnector, can be used for the intermediate-temperature SOFC. In addition, the intermediate-temperature SOFC can considerably reduce the manufacturing cost by providing manufacturers with more options in selecting the design and material of BOP (balance of plant), which accounts for about 50% of the manufacturing cost of SOFC system. Further, as the operation temperature decreases, durability increases due to the facilitated thermal cycle treatments such as start-up and shut-down.
In general, SOFCs can be classified depending on the shape of stack. Cylindrical stack is prepared by connecting cylindrical unit cells, and planar stack is prepared by stacking planar unit cells. Cylindrical stack is structurally more stable than planar one and does not require gas sealing, while it has drawbacks such as a relatively lower power density, higher manufacturing cost and poor scalability.
Planar stack can be prepared by forming unit cells and sintering them together. Although this process is simple, it is technically limited due to the difficulty in sintering various materials having different properties. Planar stack can also be formed by sintering each of unit cells separately, which process is widely studied in fuel cells.
Techniques for manufacturing the separate stack are at the level of commercialization because stacks having power of kWs have already been developed. A key point for full commercialization of the separate-stack SOFC lies in designing an optimized system and increasing the long-term stability and economic efficiency of stacks.
In the intermediate-temperature planar SOFC, CeO2-based material more ion-conductive than YSZ is used as electrolyte, nickel-ceria(Ni-SDC)-based material is used as a fuel electrode, and LSM (La0.65Sr0.35MnO3) is used as an air electrode. In particular, stainless steel (Fe—Cr-based alloy) can be used as an interconnector instead of a metal due to a relatively low operation temperature, thereby considerably reducing the manufacturing cost.
Although the planar stack is superior to the cylindrical stack in efficiency and power density due to a relatively shorter circuit route, it has the following drawbacks: (i) ductile fracture is easily caused by ceramic composites, a main ingredient in stack, and (ii) the preparation process is complicated. In particular, the development of a sealant for sealing the SOFC-constructing elements is required. When fuel gas is mixed with air at a high temperature, air-induced oxidation of fuel gas causes abrupt heat generation or explosion, thereby damaging SOFC stacks with the operation suspended. The mixture of two gases also lowers partial pressure of each gas, and thus reduces electromotive force according to the Nernst-Einstein equation, thereby preventing a normal power generation.
There have been various attempts made to develop a sealing method and a sealant that can satisfy both long-term sealing performance and material reliability. However, technical development is insufficient for the commercialization of SOFC. It is difficult to develop a sealant which meets the following requirements.
First, it needs to be sufficiently bound to SOFC-constituting elements such as a cathode, an anode, a solid electrolyte and an interconnector so as to maintain non-weakened binding region despite repeated heating and cooling.
Second, it needs to have a thermal expansion coefficient similar to that of a solid electrolyte, a cathode and an interconnector, which is 11−12×10−6/° C. (In general, the greater the difference in thermal expansion coefficient between the SOFC-constituting elements and the sealant, the more frequent the interfacial failure.)
Third, it needs to have a wetting contact angle of higher than 90° when contacting SOFC elements in order to prevent its penetration into micro-pores that can be caused by a capillary phenomenon occurred when it contacts a micro-structured electrode.
Fourth, it needs to be chemically inert without being eroded at the operation temperature of SOFC.
Fifth, it needs to survive extreme conditions for oxidation or reduction without being chemically decomposed or evaporated.
Sixth, it needs to have a specific electric resistance of higher than 104Ω·cm at the operation temperature of SOFC so as to maintain electric insulation between SOFC elements while preventing a fuel gas from being mixed with air.
Extensive researches have been conducted to use a glass material as a sealant for SOFC for it satisfies the aforementioned requirements. Initially, a glass material used for window glass such as soda-lime silicate, alkali silicate, alkali-earth silicate and alkali borosilicate glass was used as a sealant. However, these materials are inappropriate because they react with SOFC-constituting elements and tend to be leaked due to a significantly low viscosity at the operation temperature.
Borosilicate glass such as Pyrex has a much smaller coefficient of thermal expansion (3.2×10−6/20 C.) than that of SOFC-constituting elements, which results in a relatively high thermal stress. (Alkali-earth silicate)-based glass has been studied only on the composition used in relatively high temperature of 700° C. or higher. Although attempts have recently been made to use mica, a high-temperature elastomer, mainly by U.S. Pacific Northwest National Laboratory, no satisfactory results have been reported due to the structural problems of mica.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.